1. Field of the Invention
The present invention relates to signal processing using surface acoustic waves (SAW). More particularly, the invention relates to monolithic SAW storage correlator having separately addressable FET taps and having capability to process high frequency signals where the processing is controlled by low frequency analog digital signals.
2. Description of the Prior Art
Modern, high speed signal processing systems are increasing the incorporation of acoustic components. Surface acoustic wave (SAW) devices offer the advantages of high reliability, low power consumption, small size, simple construction and the ability to process high-speed signals in real time as an alternative to much larger, more complex electronic systems.
Several types of SAW devices for correlation/convolution of high-speed electrical signals are used in the prior art but all such conventional devices have limitations overcome by the present invention.
One type of prior art SAW device utilizes an air-gap technology. A piezoelectric substrate is spaced near a semiconductor material across a very thin gap. Metal electrodes attached to the piezoelectric material generate surface waves which can interact with the charge carriers in the semiconductor material via the local electric fields carried by the surface waves in the piezoelectric material. The major disadvantage of such air-gap SAW devices is the difficult and expensive fabrication of the air-gaps to the tolerances required. The present invention uses a thin piezoelectric film sputtered onto a non-piezoelectric semiconductor substrate and therefore does not require an air gap. The present invention is consequently much simpler and less expensive to fabricate than those devices employing the air-gap technology. The monolithic construction of the present invention permits taking advantage of the SAW induced interaction between the piezoelectric film and the semiconductor substrate. Additionally, monolithic SAW devices can be made in which the necessary switches, modulators, etc. required for use with SAW filters can be constructed on the same substrate.
Some prior art SAW devices have been constructed with field effect transistors incorporated into the semiconductor substrate after which peizoelectric transducers are mounted on top of the substrate or, alternatively, metallic transducers overlaid with a piezoelectric film are used. However, the devices currently available provide no means for storage of an rf modulated waveform. The present invention is a monolithic programmable device combining the high speed signal processing characteristics of SAW deices in relatively simple interconnection with a very flexible, low frequency input-output system. This enables storage of a high frequency analog waveform which may subsequently be read out of the device at low-speed, thereby accomplishing the features of a fast-in, slow out buffer memory. In one alternative embodiment, the invention acts as a high-speed, programmable filter. The filter acts on high frequency signals, yet the filter characteristics may be altered (or programmed) at low speeds.